It will be very helpful if you read this article on Wanting and Liking by Emily Yoffe at Slate Magazine before studying the chart below. The article probably oversimplifies things a bit - and most likly the chart below is very much an oversimplifyication (e.g. the roles of seratonin and other neurotransmitters are not addressed at all). But this is my best guess as to what's happening in the brain during the experience of the four jhanas - remember it's a tentative hypothesis.

1.

Quite secluded from sense desire, secluded from unskillful states -- one enters & remains in the first jhana: rapture & happiness born of seclusion, accompanied by thinking & examining.

Quite secluded from the dopamine-based wanting system (substantia nigra), secluded from any other disturbing mental activity -- one enters & remains in the first jhana: norepinephrine system activation & opioid-based liking system activation born of seclusion, accompanied by background mental activity.

2.

With the stilling of thinking & examining, one enters & remains in the second jhana: rapture & happiness born of concentration, unification of awareness free from initial thinking & sustained thinking -- internal tranquility.

With the stilling of background mental activity, one enters & remains in the second jhana: norepinephrine system activation & opioid-based liking system activation born of concentration, unification of awareness free from background mental activity -- internal tranquility due to opioids flooding the nervous system.

3.

With the fading of rapture, one remains in equanimity, mindful & clearly aware, and experiences happiness throughout ones being. One enters & remains in the third jhana, of which the Noble Ones declare, 'Equanimous & mindful, one has a pleasurable abiding.'

With the fading of norepinephrine system activation, remaining equanimous, mindful & clearly aware -- with continued opioid-based liking system activation, one experiences happiness throughout ones being. One enters & remains in the third jhana, of which the Noble Ones declare, 'Equanimous & mindful due to opioid system activation, one has a pleasurable abiding.'

4.

With the abandoning of pleasure & pain -- as with the earlier disappearance of elation & distress -- one enters & remains in the fourth jhana: mindfulness fully purified by equanimity, neither pleasure nor pain.

With the abandoning of liking generated by opioid system activation -- as with the earlier disappearance of liking generated by dopamine system activation -- one enters & remains in the fourth jhana: mindfulness fully purified by the equanimity remaining from the prior opioid activation, a neutral state now that the liking systems have calmed down.